As a semiconductor material that can be used in a transistor, an oxide semiconductor has been attracting attention. For example, Patent Document 1 discloses a semiconductor device whose field-effect mobility (in some cases, simply referred to as mobility or μFE) is improved by stacking a plurality of oxide semiconductor layers, among which the oxide semiconductor layer serving as a channel contains indium and gallium where the proportion of indium is higher than the proportion of gallium.
Non-Patent Document 1 discloses a solid solution range in the In2O3—Ga2ZnO4—ZnO system.
Non-Patent Document 2 discusses a structure in which an active layer of a transistor includes two layers of oxide semiconductors of indium zinc oxide and IGZO.